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MG MGB Technical - The saga of bad idle continues

Posted 08 January 2007 at 19:52:27 UK time
ANTHONY BASHAM, South Yorkshire, United Kingdom, t_basham@hotmail.com
Many thanks for all your advice every one of you.The saga continues but i may be a little further on i gather my first thread has not gone on it is like an epitaph
To start it i bought this car as a spares for my 76 bought sight unseen but when it came i found it rock solid to good to break only idled badly but revved ok and ran.Thought soon sought this out found out it has a dizzy from a Land Rover fitted so thought sorted WRONG i have replaced EVERY ignition part inc new dizzy,exhaust,battery,even emptied and filled tank with 4 star new pump,Done timing,dwell,set carbs,balanced them,checked that timing mark in right place,new coil every ignition check you can possibly do every thing reading OK.Still no joy rubbish tick over.SO did compression test 150PSI on every pot only went up 5 PSI wet test thought bit low for a 9.0-1 ratio
BUT i found out i have a 18V847 short motor fitted it has had a rebuild with plus .020 pistons crank overhaul oil pump this would be 9.0-1 BUT again the plot thickens it has a 12H4736 Head fitted with deep dished pistons this drops the compressions down to around 8.0-1 so the readings were OK (Then i did a Vacuum test off the inlet manifold can only pull 11Hg at idle but this reading is Rock Steady this points to either worn engine ,Valve timing out,in a SENIOR moment i had the head of to check state of engine there isn't .002 thou wear anywhere and within .001 thou of the re bore size Head crack tested valves OK so scrub that as problem .Then i have stripped the carbs down what i have found is that the needles are not the ones it should have in and the jets look different from the manual
Carbs fitted to my car are HIF4s with Breather system into carbs ID number FZX1229 these should have needles ACD fitted (THEY HAVE GOT AAU needles fitted these are for carb ID AUD493 ENGINE NUMBER 18V585Z)plus the jets look different they have four narrow grooves at the top the manual shows one wide groove i am ordering a full rebuild kit PLUS blank butterfly's so will have to wait and see if this works BUT i DON'T think this will make any difference to the very low Hg reading of 11Hg.I have checked the gauge on my other car that pulls 19Hg at idle.Does any one know how i can check VALVE TIMING without taking the cover of as i am now on my knees praying i will rebuild it as soon as all the parts come ans see what happens Sorry for the novel and hope i haven't sent you all to sleep cheers Tony

ANTHONY BASHAM

Tony; A quick way to check valve timing is to turn # 1 cylinder to TDC on the compression stroke. Both valves should be loose. Adjust both valves to say .060". Turn the engine one full turn to TDC on the exhaust stroke. Measure the valve clearance on #1 cylinder, (note if there is no clearance open the valves to a bigger gap, just be sure each valve is at the same clearance.) If you have clearance rock the engine until both valves are at identical clearances, this is easier to do with the plugs removed. When both valves are equal note the position of the TDC mark. If it's past TDC the cam is retarded, if before TDC it's advanced. You can estimate the amount based on the distance between the TDC and the 5* timing marks. You could also do this with a dial indicator with loosening the valves, but it's very easy to errors with a dial indicator unless it is operation on a level surface and is exactly vertical to the push rod. I think the valve clearance method is quicker and should get you within 2-3*, close enough for what you need.

Clifton
Clifton Gordon

Many thanks Clifton i am going to grind the valves in as i already have the head off ,then get it all back together again.I will do what you say and if the Valve timeing is OK i am hopeing against hope that the wrong needels in the carbs are going to be what the trouble was after all if this is so the old saying of dont trust diagnostic equipment rely on your own feelings and use your eyes will come to mind as i relied on the vacuum reading allthough very low being rock solid meant no problems with the carbs.(IF THE TIMING IS OK AND CARB REBUILD DOESNT WORK I AM GOING TO DONATE IT TO JEREMY CLARKSON TO GIVE IT A VERY EVIL DEATH)many thanks TONY
ANTHONY BASHAM

Do you have any idea what cam is installed? I'm just thinking that a wild profile with large overlap will give you the rough idle and reduced vacuum.
Steve Postins

Tony,

considering the short block is o.k., there are few points left that i would check. Steve allready quoted the cam, if there is a cheap fast road cam installed, this cam make a problem setting idle.
As you have done a lot of rebuilt allready to the ignition and carbs, i would check whether the break servo system has an air leak (hoses, valves and adapters too) and whether the rocker shaft is worn out (dismantle it and check for wear underneath the rocker bushes), making adjustment of the valves factically impossible.

Keep us posted, please

Ralph
Ralph

The ACD is a very lean needle fitted to later cars. The AAU is usually fitted as a replacement with K&N air filters, or even AAA.

Whichever needle is fitted, the diameter at idle is the same. The profiles start to differ after the 3rd reference, which would be at a low cruise.

If someone has "tuned" the wrong needle with a "plug chop" to run at high speed/load, where there is a big difference between ACD and AAU, then the idle mixture will be way out, and it won't idle.

The book way to tune the SU is at idle, get this right and the needle profile should sort out the fuelling. Any (spring fitted) .090 needle will fit, and since they have the same diameter at idle, the carb settings (jet height) should remain the same, so a rolling road tune reveals where the fuelling is wrong, and the operator chooses the correct profile, fits it, and the job is done.

You might like to get hold of SUSEARCH an excel spreadsheet written by W.Konrad Crist.
This allows you to compare needle profiles and see graphically where there is a difference.

http://homepage.mac.com/crscapps/CSA/Page1/Page1.html

The correct way to tune carbs is on a rolling road of course, but the factory fitment should give a reasonable compromise.
Martin Layton

Many thanks for all your comments.
Re Martins i agree with you completely .I don't think doing any thing with the carb's is going to do any thing to make it idle better its just wishful thinking on my part but i am not that big headed to say i am beaten, because when setting the carbs up a 1/4 turn makes a lot of difference to the running and you find worn carbs or really badly set up carbs or if you have air leaks you have to turn mixture screws many turns to make any alterations , again YES i agree a rolling road is the best way to tune an engine BUT we are talking about a low compression B series engine here and just the Normal way of setting simple SU carbs should work .Re the needles again i can see in just the Haynes manual that they should work and to be honest i should be able to put ANY HIF4 carbs from another MGB on my car and get it to idle and run perfect BUT IT WONT(PLEASE don't take this the wrong way as i thank everyone for their advice )I am a retired Classic car restorer and have worked on all types of cars in 40 plus yrs and have set up triple and twin,carbs without the problem's i am having with this car and have the correct equipment to do the job .I am no tuning genius i am an old fashioned mechanic but in 40 yrs have not been beaten with the older type of car but there is a lot i don't know and if someone has had the same fault i will gladly listen,THE big problem is that when we buy old cars we never know what has been done take this one wrong block and pistons for year of car.Wrong cylinder Head,Wrong dizzy fitted,should have overdrive gearbox no overdrive so its then when a problem such as this happens its a night mare.If i hadn't taken the head off i wouldn't know it had the deep dished pistons and the valve sizes,plus with the low compression head fitted the ACD needle should work fine.

Re-Ralph and Steve, many thanks for your advice ,I didn't really want to take the cam out due to me having bad health problems but it maybe my last resort to solving the problem regarding AIR leaks i have got the servo pipe plugged done all the normal checks for air leaks plus it doesn't show an air leak on the Vacuum test have also done the old fashioned tests Soapy water,sprayed penetrating oil all over the inlet manifold and carb spindles while running plus as these are both now off the car i have done a crack test on both manifold and head both OK .The rocker shaft i have in bits as we speak and it is like new plus i have even got the buckets out they show no wear.Re carbs i have now got them in bits no wear on spindles poppet valves look perfect done drop test on pistons OK so it looks like i am going to have to take the cam out if only to see for myself if its the correct one
I really do appreciate the time and trouble that every one has gone to trying to help me out even though it looks as if i am not taking notice of some of the answers as you can see i am no Muppet but i am human and have done every test possible if it was an electronic state of the art car with all the gizmo's on i could understand it kind regards to every one TONY







ANTHONY BASHAM

Stick with it Tony! It's going to be something simple; it always is. If you have a dial test indicator to hand, I'd be tempted to map the cam, without removing it. Shouldn't take long to get an idea of what's in there and may at least rule out something else.
Steve Postins

Or maybe borrowing a set of carbs from a friendly club guy in your neighborhood.

Cam timing?

Substitute a different coil?

Is charging voltage ok at idle?

Tell us what it was when you find it.
Tom

Tony,

a friend from the local MG club once had a problem with one of the core plugs, sealing the cross over connection of the inlet manifold an other one had problems with the idle due to the head was skimmed down too extreme.

Only want to give you this Information for further checking.

Steve mentioned to check the camshaft duration and the LSA with a dial gauge. I found out it is more comfortable to use two of them, each on a magnetic stand, the job is done much quicker this way and there is less calculation necessary.
BTW, you should know, that early (before 18V) engines had a different cam drive that was also positioned @ 4 deg. advanced to the later 18 V verion!

The early gears and chain were duplex, the later ones simplex, while the chain tensioner remained unchanged.
Some engine reconditon workshops are not afraight to mix early and late parts during a rebuilt, even differnt and used pistons im MG engines came my way within the years!

Ralph
Ralph

One or two thoughts from the Colonies.

11Hg of vacuum is not nearly enough. It should be in the 17 - 19 InHg range. This in itself will cause disasterous idle characteristics. 1st Most probable cause is a vacuum leak. Fairly easily tracked down with an unlit propane torch. Start it up and squirt a bit of propane in a carb inlet. The REV's should increase. Then start around the running engine spraying unlit propane. If there's a vacuum leak the propane will be sucked in and thr REV's will rise immediately.

Second is timing. If I recall correctly, the vibration dampner on the B is 3-piece. Steel inner disc, a rubber band, and a steel disc around it all. The timing mark is on the outer ring. I have seen in the past a severe backfire changing the position of the outer ring on the inner. No easy check except for making SURE #1 piston is at EXACTLY TDC (either stroke) and verifying that the timing indicators are properly aligned on the block and hormonic balancer.
tom

Just a thought about the cam. If it had a wild profile in, it to rev it's nuts off.
Whats the performance like?
Tarquin

should say,
Just a thought about the cam. If it had a wild profile in, it should be able to rev it's nuts off.
Tarquin

On the assumption that the cam is OK this looks very like retarded ignition. The situation re the markings on the damper become irrevant if you advance the timing as far as you can so that you just get pinking and no more when accelerating hard from 20-40 mph in 4th gear. Both my Bs give an idle vacuum of about 17/18"hg which I would have thought was low and would have expected 21" but there again it depends very much on the cam profile. By the time my revs rise to about 1200 I get exactly 21"
Iain MacKintosh

Again many thanks one and all for your replies
Re Tom i have changed Coil got a sport one on now the correct voltage and tested all the electrics plus new battery,yes its charging at nearly 14V and the volt readings at the coil spot on.with the head off i have cheacked the timing marks are spot on,and i have done air leak test the old way soapy water and WD.Plus with a Vacuum gauge test air leaks show up straight away the LOW Hg is Rock steady no RPM change as for cam got to do that yet

Re Tarquin I will be very suprised if the Cam is a tuned one as dont get me wrong over 1200rpm the car sounds bang on no pops,back firing ,missing etc but it doesnt even sound as throaty as my other 76 MGB and isnt as quick fair enougth i tested it without all the new stuff on

Re IAN I Agree with you 100% you can forget about timing marks vacuum ETC when you are doing a VACUUM test from a good inlet manifold source moving the distributor to Advance and Retard the Ignition SHOULD show on the gauge as higher or lower readings my gauge is a snap on one reading in single units i am lucky if i get plus or minus 2Hg while moveing the dizzy(THIS IS THE MAIN THING I JUST CANNOT UNDERSTAND)and the daft part is that as soon as i rev it to over 1300rpm the Hg is correct and does exactly what it should on snap throttle,and over run,plus i have tested the gauge on my 76 model and it pulls approx the same Hg on there as yours at idle
Finally Ralph if you havent all fallen asleep cheers for your tips i will recheack the head and look harder at the manifold plus i agree with part swaps take this car the head is not the usual one fitted to UK MGB's its a LC head according to data to be fitted with dished pistons which i have in my block re cam as i have said i am no tuning wizard but i have had tuned engines and this one does not sound or performed like a tuned lump i thought of this because in the pinto days you had a 2litre block but put a 1600 head on with Granada pistons then put better carbs on at least thats what my old memory serves me and i thought is this why they have put this head on is it a tuneing mod but i dont think so any way cheers to every one i am sure with all your help between us we will prevail TONY



ANTHONY LINDSEY BASHAM

Tony,

cars fitted with LC pistons and the small combustion chambers in the head also used manifold vac for the distributer (most USA versions had tis setup in the 1970's). UK and European export cars used ported vac. from the rear carb and therefore had other dizzys fitted.

When I bought a 1975 roadster in Alabama in the erly 1990's, it came with Stromberg carb and cat and all the other stuff, i took off the engine immediately and installed HS 4 carbs but first used manifold vac and it was aughful running.

Things turned good using a D25 from a Mini 1275 (also used on MG 1300 saloons) and connecting the vacuum to the port of the rear carb, pluging the manifold pichup.

May be you can check this on your car too.

Cheers

Ralph
Ralph

HI Ralph i don't know if you have the same info as me because its a mine field out there with these British Leyland cars.I have full spec sheet's for both USA and the UK Markets and a spec sheet on the heads and blocks the big problem is half of the time they don't match up , I believe my car has the LC head fitted to be used with dished pistons on re spec sheet,the Block could be early or late BUT it has got dished pistons fitted i will give the numbers again and if you or any one has any comments i would be intrested its funny you talking about putting a 25D dizzy on as the USA spec gives my head ,piston ,block combination as using a 25D BUT the UK spec for my engine number gives the 45D the trouble i have is with the head i have on and the dished pistons what do i use a 25D takeing vacuum of carb or a 45D taking vacuum of the manifold,BUT then it gets better because to be honest if you block the vacuum unit off which is what a lot of people do you can run either dizzy you just have to reset idle because when using 45D and you use the vacuum off the manifold as you should it is on full advance at idle you then have to turn the idle
screws down (All this is for is fuel economy)Where when a 25D dizzy is used Vacuum advance does not come in until the engine is revved so you have to open the idle screws for it to idle at the correct speed in theory you should be able to use either distributor if you blank vacuum of and set the timing and idle speed re car data
MY CARS DETAILS AGAIN
BLOCK IS A 18V 847F BUT HAS DISHED PISTONS FITTED
HEAD IS A 12H4736 with 1.562 inlet and 1.343 exhaust
DIZZY 45D 41610
CARBS are HIF4 needle ACD breathers into carbs
cheers again TONY
ANTHONY LINDSEY BASHAM

There is no fundamental difference between carb and manifold vacuum. When the UK eventually changed from carb to manifold in late 76 the distributor (and engine) remained the same. The only difference between carb and manifold is at idle where carb gives no vacuum and manifold gives high vacuum (which is why it has to be disconnected and plugged for timing adjustments). As the throttle starts to be opened the carb vacuum rapidly rises to meet the level of manifold vacuum. As the throttle is opened further they both gradually fall. Zero vacuum at idle makes for easier starting which is why they were plumbed that way in the pre-emissions days. High vacuum at idle makes for lower emissions at idle, hence the change as emissions regs were tightened up. US spec cars got loads of different distributors and engines to meet continually changing emissions requirements - according to my information no less than 13 different types *after* they changed to manifold.
Paul Hunt 2

In my experience, you can get the car to idle at pretty much any ignition setting you choose - even way, way out. Vacuum advance isn't going to make any difference.

What do you mean by 'bad idle'? Is it rough/lumpy, or is there a misfire, is it hunting?

I've got a 'hot' cam and you can get the engine to idle smoothly, albeit at higher revs eg 1200rpm. As you reduce the revs it sounds like an old single/twin motorbike engine - "burum, burum, burum".

An easier way to check the cam without much dismantling is to measure the lift at the top of the pushrod or at the valve. If the lift is more than standard then perhaps the cam is not standard as 'hot' cams usually have increased lift. If the lift is standard, cam is probably standard.

I've got an SU manual, says that no groove on the jet makes it a 0.09" and one groove makes it a 0.1", no mention of four grooves.

Another thought is the third hole in the carb/filter mounting flange - could be blocked by a home-brewed gasket.

Another long shot is bent rods - I knew someone who drove through a flood and filled the engine with water, bending the rods. Also have a brother who bent the rods when the head gasket blew big time.

Last but not least (and I know this sounds daft), hopefully you are not using the rev counter in the car to check the revs?

Neil
Neil Lock

Neil cheers for your comments would it be possible to send a scan of the Manual for the SU carbs also what method do you use to see if the valve timing is OK .At the moment i have every thing stripped and apart from a bit of wear in the Rocker Shaft i can still find nothing wrong NO wear in the BORES or the HEAD i have sent for a recon shaft assembly new push rods and buckets plus a carb rebuild kit yes i have all the equipment to set engines up Analysers ,timing light compression tester,vacuum tester carb balancer and c/o meter this is the trouble i have renewed and checked Everything still am no wiser to the problem.I have another MGB a 76 that ticks over like a watch this one has a splash to the idle plus it keeps going out of tune i can set the idle get it to idle ok at 900 revs switch it of then it runs like a dog i then readjust mixture i know you will say air leack or fuel flooding but i can assure you it has neither when i rebuild the carbs i am going to change the bimetal part that the jets fit to and see if that works cheers TONY
A Basham

Tony,

is there a fuel filter in the line just before the carbs? If not, there should be one fitted. If there is one in place, is it conterminated?

Ralph
Ralph

Tony,

Doesn't splashy misfire mean too weak?

Slop in the spindles/linkages can make it difficult to set the idle. Could also be the throttles not closing properly or one not closing properly due to the way the linkages are set up. Have you got 'extra' throttle return springs? Or the poppet valves on the throttle discs? You say you have sent for the carb rebuild kit; I thought you had already done that. If not then there could still be a problem with the carbs. I don't know anything about HIF carbs though...

It is easy to check the valve timing when the engine is apart, less so with the head on as you cannot accurately determine TDC. You need a cam timing disc and a dial test indicator (DTI). You fit the cam degree disk to the front pulley and rig up a pointer. Then you find TDC on number 1 cylinder using the DTI, going backwards and forwards past TDC to find the mid point. Then set the cam degree disk to zero and then find when the inlet valve starts to open ie pushrod starts to move upwards, should be 16deg before TDC according to my book. That's if you've got a standard cam.

To be frank, if you haven't refurbished the carbs yet then I'd be voting for a carb problem. But if you're engine is in pieces you may as well check the cam timing. If it's a standard cam and the dots line up, then I'm thinking that cam timing is not going to cause the problem.

Neil
Neil Lock

Hi Neil thanks What i want to know is will the method you have given tell me if the valve timing is correct WITHOUT taking the cover off to see the spots are in line, as i really dont want to strip the engine down any more due to my health as you have to take the covers of ,the rack etc
The carbs no they are not stuck the butterflies are shutting correctly,they are in pieces AGAIN getting a full rebuild and Every O ring Seal looks perfect NO wear on spindles and yes i do have the poppet valves but they seem ok cheers TONY
A Basham

This thread was discussed between 08/01/2007 and 18/01/2007

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